Components are made from flat products of the type discussed here, which are subsequently coated with one or multiple coats of paint, in order to protect them on the one hand from possible corrosion and on the other hand to optimize their visual appearance. The quality of the visual appearance is judged in this case among other things by how far the surface texture of the respective metal substrate affects the surface of the paint finish.
Particularly high demands are placed on the appearance of the surface of automotive body panels visible from the outside.
In practice, the demands made on the paint finishing of body components are met by the application of multi-coat paint systems. These paint systems usually comprise at least one so-called “filler coat”, whose object among other things consists of adjusting any unevenness, which might exist on the surface to be coated.
The cost associated with the application of multi-coat paint systems onto sheet metal is substantial. Modern painting processes achieve cost savings by omitting the filler coat. These processes are being used more and more by the automotive industry. In this case, the total thickness of the paint system is substantially reduced, so that the metal substrate could show up in the finish of unsatisfactory sheet metal.
A further criterion for assessing the suitability of a metallic flat product for producing body components is its behaviour when formed into the respective component. Also, this is crucially influenced by the surface texture of the respective flat product. Thus, the cavities existing on the surface of a metal sheet, during deep-drawing for example, form pockets, in which a lubricant applied onto the metal sheet before its forming or injected into the respective die can accumulate. The load-bearing capacity of the lubricating film formed by the respective lubricant in this case directly depends on the configuration and distribution of these cavities.
Various attempts to structure the surface of metal sheets so that after paint finishing they possess an optimized appearance are known. Examples of these attempts are indicated in Japanese Patents JP-A 63-50488 and JP-A 1-293907.
The regular surface textures described in these two publications of Japanese patent applications are characterized by cylindrical, punch-type elevations, which are encircled by a groove-like recess and project from an otherwise even surface.
In accordance with JP-A 63-50488, the plateaus of the peaks are located approximately 2-10 μm above the soles of the valley regions existing between the elevations. At the same time the combined percentage of even plateaus of the peaks and even surfaces of the average flat regions existing between the soles of the valleys and the peak plateaus amounts to 20-90% of the total surface area.
In JP-A 1-293907, it is also stipulated that the percentage of flat regions between the regularly arranged peaks with circular cross section should assume at least 85% of the sheet metal surface, that the depth of the valleys surrounding the peaks extending from the flat regions should amount to at least 4 μm and, according to a frequency analysis of the sheet steel surface geometry, the intensity of the wavelength portions of the wavelengths λ, which lie in a range of 585 μm<λ<2730 μm amounts to 0.6 μm2 at the most.
The metal sheets constituted in accordance with the two Japanese patent applications, in the painted state, are to leave behind an extremely vivid impression. However, the requirements preordained for this presuppose strictly deterministic surface textures. More particularly, the high intensities, permissible according to JP-A 1-293907, in the wavelength portions specified there, only arise in the case of deterministic structures with high periodicity.
In practice, however, it is evident that regular surface textures, obtained in accordance with the prior art described above, can only be produced with the necessary reliability under difficulty. This applies especially if the substrate to be processed concerns galvanized sheet steel.